Optical methods for measuring distances are known. Thus, distance measurements are carried out electrically, by measuring the transit time of a light pulse reflected from a target. Equipment operating on this principle is mainly used in land surveying. For measuring short distances in the centimeter (cm) and decimeter (dm) range such a method is complicated because the short transit times, in the nanosecond range, require rapid electrooptical and electrical switching elements.
The triangulation method is more suitable for measurements in the dm range. It is based on producing a light spot on the target by a sharp light beam emanating from the measuring apparatus. This light spot is imaged on at least one light detector, which is inclined (laterally offset) with respect to the primary beam. The distance between the measuring apparatus and the object can be determined trigonometrically from the angle between the primary beam and the light spot-image spot connecting beam.
In order to ensure accurate angular determination in triangulation methods, the light spot on the object or target surface must be sharply imaged by means of a lens on the light detector, e.g. a diode array. In the case of a fixed lens setting, this permits high measuring accuracy only in a narrow distance range, unless the optics are constantly readjusted in an iteration process, which involves considerable extra effort and cost.
A further optical distance measuring principle is based on the measurement of the diffuse light scattering of a light spot produced on the object by means of a finely focussed light beam. Assuming that purely diffuse scattered light emanates from this light spot, the radiation at right angles to the object surface is approximately homogenous (Lambertian radiation). Two light detectors are arranged with different spacings along said beam, which is optionally focussed by means of a collimating lens. Thus, it is possible to determine from the measured illuminations the divergence or convergence angle of the beam. This also gives the sought object distance on the basis of the "inverse square law", i.e., the fact that the radiation intensity of a point source is inversely proportional to the square of the distance.
Methods of the above type are known, as is apparent from the patent specifications referred to hereinafter:
In U.S. Pat. No. 3,719,421, J. L. Poilleux and J. Tourret describe a method in which a light spot is produced on the object by means of optics and is subsequently imaged. Two diaphragms with detectors arranged beyond or in front of the image point permit a distance determination within a limited range by subtraction of the detector signals This method is suitable for accurate distance determination within narrow limits, but not in general terms for distance measurement. Outside of the aforementioned design range it is even ambiguous to associate the detector signal with the object-measuring apparatus spacing. Reference will be made concerning the detailed construction of the apparatus hereinafter in connection with FIG. 2.
A necessary criterion for the reliability of this method is the existence of diffuse back-reflection, but if reflection occurs in addition to diffuse scattering, measuring errors occur. The above U.S. patent describes measures which increase the corresponding measuring accuracy.
A further method, in which the distance measurement is based on the "inverse square law" is described in West German Patent No. 2 703 463 by E. H. Mehnert. In the latter the distance from a point light source is determined by measuring the illuminations of two surfaces located at different distances from the light source.
Three different apparatuses are given for carrying out the method. A first apparatus is based on the measurement of a large-area illuminated surface using two light detectors arranged behind teleoptics and which are located at different distances from the object. Although the teleoptics permit the sharp imaging of only a small area of the object on the detector surface, when the object is displaced the object spot is only unsharply imaged on the detector, but the illumination of the image is not changed, unless the object is very small, i.e., a small light spot. Thus, the apparatus according to claims 2 and 3 of the aforementioned patent cannot function.
In a second apparatus use is made of an infrared the object. The, radiation of this light source is measured by at least one light receiver. When the emitted intensity of the light transmitter is kept constant, it is possible to directly determine the distance from the apparatus to the infrared diode on the basis of the "inverse square law". If there are two light receivers, which can e.g., be realized by means of semi-reflecting mirrors, then the distance determination is not dependent on the radiation intensity of the light source.
The relationship between the measured illuminations of the light detectors and the distance is quadratic and not linear. By means of modern digital signal processing equipment, it is not difficult to resolve the "inverse square law". However, unlike the case of linear behavior between the measured quantity and the distance, the measuring error is not constant but, as a first derivation, is linearly dependent on the divergence from the desired value.
In addition, said second embodiment of the apparatus suffers from the disadvantage that the infrared diode must be located precisely in the optical axis of the light receiver. Otherwise, the light detector signal can no longer be unambiguously associated with the object distance.
Taking account of this latter fact, according to a third embodiment the light source, particularly a laser, is arranged directly in the optical axis of the measuring apparatus and, on the basis of FIG. 3, a detailed explanation will be given thereof hereinafter. However, it is pointed out here that also in the case of this third embodiment the link between the detector signals and the object distance is non-linear, which significantly influences evaluatability and measuring accuracy.
A third method based on the "inverse square law" forms the subject matter of German Patent application No. P 37 43 194.3 of Dec. 19, 1987. According to the latter, the spacing or distance information is also taken from the illumination of two light detectors at different distances from the object. The diffuse radiation emanating from the illuminated light spot is focused by means of optics. However, focussing does not take place two-dimensionally by means of an objective or lens, as described by J. L. Poilleux and J. Tourret in U.S. Pat. No. 3,719,421 but instead takes place only one-dimensionally by means of a cylindrical mirror. The light detectors, one of which is located in the focal line of the cylindrical optics, determine in one dimension the full light beam entering the apparatus. This structural embodiment differs significantly from West Germany Patent No. 27 03 463 of E. H. Mehnert, where there is no one-dimensional focusing and the position of the detectors is not specified. The new method gives a linear dependence of the distance to be measured on the quotient of the measured detector signals. The apparatus on which this method is based is explained in detail in connection with FIG. 6.